Component Project I: Position-Specific Functions of no tail (ntl): Untangling the Pleiotropy of the no tail Mutant Principal Investigator: David Jonah Grunwald; Co-Investigator: H. Joseph Yost T-box genes encode related transcription factors that regulate development of organs and regions of the body. Of the >20 known members of the family, mutations in five genes have been linked to human congenital disorders. Despite extensive study, the phenotypes caused by T-box mutations in humans, mice, or zebrafish are complex and poorly understood: multiple tissue types are affected, and in no case has a unifying cellular defect been identified that accounts for all developmental defects of a mutant. We propose that pleiotropy is an intrinsic feature of T-box genes that needs to be deconvoluted to understand the cellular basis of T-box mutant phenotypes. Here we test two hypotheses that that would account for why loss of a Tbox transcription factor frequently results in a multiplicity of cell type defects. First, we propose that individual T-box transcription factors have different sets of downstream target genes in different regions of the embryo, thus explaining the recurrent pleiotropy of T-box mutants. Second, we propose that one factor that contributes to diversification of T-box factor function is interaction with additional T-box transcription factors. Hence loss of one T-box transcription factor affects the function of other T-box factors that continue to be expressed in a developmental field. Component Project I analyzes the downstream functions controlled by no tail, the zebrafish orthologue of the vertebrate Brachyury/TJ-box gene, no tail is expressed throughout the mesoderm and required for development of the notochord, the posterior mesoderm, and the tail organ, Kupffer's Vesicle. The goals of Component Project I are: 1) identify the position-specific transcription activities of the No Tail protein; 2) analyze how interactions with additional T-box factors contribute to the position-specific activities of No Tail; and 3) test the idea that distinct direct target genes of NTL carry out independent position/tissue-specific developmental functions of no tail.